1. Field of the Invention
The present invention relates to digital subscriber line transmission systems, which allow, in particular, high speed communication on twisted pair telephone lines based on discrete multitone transmission (DMT). The invention relates more specifically to a far-end crosstalk (FEXT) canceller for compensating the crosstalk signal induced by modems located at the far-end of such a transmission system.
2. Discussion of the Related Art
FIG. 1 schematically shows a modem, comprising a transmission (TX) and a reception (RX) section in a conventional DSL transmission system using discrete multitone. A serial stream of data X is provided to a mapper circuit 11 mapping each data into a symbol of a constellation, for example of a QAM (Quadrature Amplitude Modulation) constellation. The mapped values are then transformed into a set S of N components by a serial to parallel converter 12, each component of the set being considered as a frequency domain coefficient. This set of frequency domain coefficients, hereafter also called DMT symbol, is provided to an inverse fast Fourier transform (IFFT) circuit 13 which generates a time domain block of samples and is followed by a parallel/serial converter (P/S). This time domain block is therefore the sum of N sinusoidal subcarriers of different frequencies, the amplitude thereof being determined by the corresponding frequency domain coefficient received by the IFFT circuit.
Each time domain block is cyclically prefixed (cp) and suffixed (cs) in a block 19 to eliminate or at least attenuate the Inter Symbol Interference (ISI) and the Inter Carrier Interference (ICI) caused by the channel, and is transmitted onto a telephone line 10 through a hybrid line interface 18. The line interface 18 also receives incoming time domain blocks from another modem connected to line 10.
At the receiving side, the incoming time domain blocks from line 10 are provided to a fast Fourier transform (FFT) circuit 14 through a block 19′ that deletes the prefix and suffix and a serial/parallel converter (S/P) which calculates the N frequency domain coefficients for each block. The N frequency domain coefficients are then provided to an equalizer 15 which compensates for the attenuation and phase shift incurred by each frequency component. The equalized values are then serialized by a parallel to serial converter 16 into a stream of N complex numbers R(fj) and then processed by a demapper 17 attributing to each R(fj) the symbol Ŝ c of the constellation which comes closest thereto. The demapper 17 further outputs the digital word {circumflex over (X)} c associated with the selected constellation point Ŝ c.
FIG. 2 schematically shows a DSL transmission system including a central office 20 communicating with a plurality of end-users over telephone lines 25, 26. The modems M1, M2, Mc(1), Mc(2) have the structure represented in FIG. 1. The end of a telephone line connected to a modem of the central office is called the line termination (LT) side while the end connected to a modem of an end-user is called the network termination (NT) side.
Ideally, such a DSL transmission system allows the whole frequency band to be used for simultaneous full-duplex transmissions. However, in practice, different sources of noise disturb the transmissions and impede proper reception of data.
For a given modem, three different sources of noise can be distinguished as illustrated on FIG. 2:                the self-echo, i.e. for a given modem, the parasitic signal from the transmitter TX leaking to the receiver RX through the hybrid interface;        the near-end crosstalk (NEXT) arising from signals in adjacent telephone lines 25, 26 with opposite transmission directions. More specifically, in the present example, the NEXT generated at the modem Mc(1) is the parasitic signal received by this modem from the modem Mc(2). In this instance the NEXT is called NT-NEXT because the modem Mc(1) is located on the NT side. Reciprocally, the NEXT generated at modem M1 by the modem M2 is called LT-NEXT;        the far-end crosstalk (FEXT) arises from signals traveling along the same transmission direction in adjacent telephone lines. More precisely, in the illustrated example, the FEXT generated at the modem Mc(1) is the parasitic signal received by this modem from the modem M2 located on the opposite side, due to the coupling between the telephone lines 25 and 26 sharing a common binder. In this instance the FEXT is called NT-FEXT because the modem Mc(1) is located on the NT side. Reciprocally, the FEXT generated at modem M1 by the modem Mc(2) is called LT-FEXT.        
Echo-cancellers for canceling self-echoes are known e.g. from U.S. patent application Ser. No. 09/410,636, filed Oct. 1, 1999 and entitled DSL TRANSMISSION SYSTEM WITH ECHO-CANCELLATION, which is incorporated herein by reference.
There is also known from U.S. Pat. No. 5,887,032, which is incorporated herein by reference, a canceller for canceling out the NEXT interference in an ADSL transmission system on the LT side. This canceller operates in the frequency domain and assumes, for a given subcarrier or tone, that the NEXT interference is proportional to the symbol value emitted by the modem transmitting on the interfering channel. The latter value is scaled by a given coefficient and subtracted from the symbol received by the modem suffering from the NEXT interference.
Both self-echo cancellation and LT-NEXT cancellation are possible because the signal transmitted by the same modem (in the case of the self-echo) or by a neighboring modem of the central office (in the case of LT-NEXT interference) is directly available.
FEXT cancellation is however intrinsically more complex than NEXT or self-echo cancellation because the modem transmitting over the interfering channel is now located on the far-end side and the actual values of the interfering symbols are therefore not known.